1. Field of the Invention
The present invention relates to two-way pagers and, more particularly, to an apparatus and method for efficiently charging a rechargeable or secondary battery of a two-way pager.
2. Description of the Related Art
A conventional two-way pager includes a main or primary battery for supplying a power source voltage to each element of the pager except a transmitter, and a rechargeable or secondary battery for supplying a power source voltage to only the transmitter. An alkaline battery of 1.5V is generally used as the primary battery, and a nickel-cadmium (Ni--Cd) battery of 3.6V is generally used as the secondary battery. The rechargeable battery of 3.6V consists of 3 cells each having 1.2V and constitutes a constant current source as an alternating current (AC) adaptor in order to supply a power source voltage to a mobile telecommunication terminal.
FIGS. 1A and 1B are graphs respectively illustrating discharge and charge efficiency of the rechargeable battery in the two-way pager with respect to a charge input (or charge ratio). FIG. 1C is a graph illustrating charge and discharge voltages of the rechargeable battery in the two-way pager with respect to a charge input.
As shown in FIG. 1B, the charge efficiency is satisfactory between the charge ratios of 25% and 100%. However, if the charge ratio is over 100%, the charge efficiency does not increase, but rather, rapidly decreases. The point at which the rechargeable battery voltage drops after a charge input of 120% is denoted as a full charging index.
The way in which sufficient charging is performed per unit cell is important. For example, in order to prevent overcharging of the rechargeable battery or a memory effect, full charging and full discharging processes are repeated. To this end, a charger having a microprocessor is employed. The point at which full charging has been achieved is referred to as the cut-off power source voltage. That is, the change .DELTA.V in a cell voltage is about 20 mV on a charge voltage curve 20, as shown in FIG. 1C. In order for the charger to fully charge the secondary battery, a charge current of 140%-160% or more of the rechargeable battery's capacity should be applied.
In a mobile terminal such as a cellular phone, since an AC power source is used as the charger, the rechargeable battery is not significantly affected with respect to charge efficiency. However, since a two-way pager employing a Ni--Cd battery for supplying a power source voltage to a transmitter charges the Ni--CD battery by using a replaceable primary battery instead of an AC adaptor, the charge efficiency and lifetime of the rechargeable battery are reduced.